Siou - when you checkout, there's a spot to enter a coupon code:
" Will you be using a coupon or gift certificate today?
If you have one, enter your gift certificate or coupon code "

Enter the code, and go to the next step. If the code worked, there will be $3 deducted from your order - complete everything else as usual.

One thing to watch out for: if you navigate away from the checkout page, you'll have to enter the coupon code again. So make sure you've put everything you want in your cart and used the STP link of the person you want to point before starting the checkout process. :-)